The present invention relates mainly to a method of charging a rechargeable battery such as nickel-cadmium (Ni--Cd) battery or nickel-metal hydride (Ni-MHD) battery, which shows a decrease, .DELTA.V, in voltage from the peak voltage after it is fully charged.
In order to charge a rechargeable battery, it is important to detect the full-charge state and to stop supplying charging current on the completion of full capacity charge (100%), because overcharging causes reduced battery quality on one side, and incomplete charge, i.e. less than 100% capacity charge and leads to a shorter service time on the other. But it is not easy to charge a rechargeable battery to the full capacity under various conditions.
A rechargeable battery such as Ni--Cd battery or Ni-MHD battery shows a decrease, .DELTA.V, from the peak voltage when the charging continues after it has benn fully charged. This characteristic is utilized to detect the full-charge state of a rechargeable battery.
Under normal conditions, the full charge state can be detected accurately by detecting the peak voltage or a decrease, .DELTA.V, in voltage. The rechargeable battery with an unduly low output voltage caused by overdischarge shows charge voltage characteristics different from normal characteristics, when it is fast-charged. Some rechargeable batteries with unduly low output voltage are no longer usable because of an internal short circuit. In order to discriminate batteries between usable ones and unusable ones, weak charging current is supplied in the first step of fast-charging until the output voltage reaches a predetermined voltage, for example 1V, then fast-charging begins.
FIG. 1 shows a voltage curve in a fast-charging process of a battery with overdischarge. As shown in the figure, the charge voltage shows a sharp rise immediately after the fast-charge process begins following the preliminary charge then the charge voltage decreases. Thus a characteristic curve shows a decrease, .DELTA.V, in voltage after it reaches a peak value. When the charge voltage changes in the similar curve pattern, the battery charger determines the completion of the charging process and terminates the charging process. Thus charging process is terminated before the battery has been fully charged. The rise to the peak and decrease, .DELTA.V, in the charge voltage is remarkable when the battery temperature is low.
In order to overcome the drawback, a new method has been proposed in which detection of the voltage decrease is nullified for a certain period of time after the quick charge process begins following the preliminary charge. But the method has the difficulty of setting the time period during which the voltage detection is nullified. In the case where an unduly long time period is predetermined, overcharging which tends to reduce battery quality often occurs due to the failure in detecting the full-charge state.
In the case where an unduly short period time is predetermined, charging is no longer possible for the battery which shows the voltage decrease, .DELTA.V, before the full-charge state has been reached.
The present invention has been developed to overcome these drawbacks. The main objective of the present invention is to propose a method of fast-charging of overdischarged rechargeable batteries capable of accurately charging to the full-charge state.
The above and further objectives and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.